Various phases of research in thyroid function and iodine metabolism are being pursued as follows: 1. Purification of thyroid peroxidase and determination of its chemical and physical characteristics. 2. Determination of the role of thyroid peroxidase in iodination and in thyroxine biosynthesis. 3. Elucidation of the mechanism of iodination of thyroglobulin and of the mechanism of iodothyronine biosynthesis. 4. Elucidation of the mechanism of action of the clinically used antithyroid drugs. 5. Elucidation of factors responsible for the failure of metamorphosis in Mexican axolotls.